References considered to be relevant as background to the presently disclosed subject matter are listed below:                1. U.S. Pat. No. 8,004,012 which share the inventors and the assignee of the present patent application        2. “The effect of absorber doping on electrical and optical properties on nBn based type-II InAs/GaSb strained layer superlattice infrared detectors” by S. Mysers, E. Plis, A Khoshakhlagh, H S Kim, Y Sharma, R Dawson, S Krishna and A Gin, Applied Physics Letters 95, 121110 (2009)        3. U.S. Pat. No. 7,795,640 which share the inventors and the assignee of the present patent application        
Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.
Photodiodes are widely used for sensing light radiation. There are many applications in which the level of the light which is required to be sensed is very low, and therefore the sensitivity of said photodiodes is acritical requirement.
It is well known in the art that the signal-to-noise ratio which can be obtained from photodiodes (and from many other electronic components) is limited by the level of the “thermal noise”, which in turn is related to the temperature of the component. The term “dark current” is commonly used in the art to define the current flowing in a photodiode during a total dark condition. The signal-to-noise ratio in infra-red photodiodes is conventionally improved by cooling the component, in many cases down to very low temperatures close to 77 K, or even lower. The means for cooling and maintaining such a low temperature in photodiodes, however, are cumbersome and expensive, and in any case can reduce the noise down to a limited value.
The dark current is generally composed of two main components. The first component, hereinafter referred to as “the diffusion dark current” is due to the thermal excitation of carriers across the complete energy bandgap of the photodiode sensing material. As said, the level of this current can be reduced by means of cooling the component. The second component affecting the level of the dark current is known as the “Generation-Recombination” current (hereinafter “G-R dark current”). The level of the G-R dark current can also be reduced by cooling, but at a slower rate of reduction to with temperature. At low temperatures, where the level of the diffusion dark current is reduced sufficiently, the G-R dark current generally becomes the most dominant component of the dark current. There have been made many efforts in trying to reduce the level of the thermal noise. However, there are not known many of such efforts for reducing the G-R current.
The maximum operating temperature of a solid state infrared detector is usually determined by its dark current, which increases exponentially with temperature. In standard mid-wave infrared (MWIR) photodiodes operating under conditions of background-limited performance (BLIP), this dark current is usually produced by so called generation-recombination (G-R) centers (also known as Shockley-Read-Hall traps) in the depletion region of the device. A reverse bias applied to the diode activates the dark current for these G-R centers, which provide energy levels close to the middle of the bandgap. As a result, the amount of thermal energy needed to excite an electron out of the valence band or into the conduction band is approximately halved. Electron-hole pairs are generated that are immediately removed by the electric field of the depletion region.